Communications systems employing optical fibers are well known in the art. These systems typically transmit data by using a light source, such as a laser, to emit pulses of light onto a waveguide. The waveguide, often implemented as a glass fiber, transmits the light pulses to an optical receiver that senses the pulses of light and provides a corresponding output signal (typically an electrical signal) to a receiving system.
Optical communications systems may span large geographic regions, or they may be implemented within single electronic components. Recently, vertical cavity surface emitting lasers (VCSELs) have been recognized as being useful in small-scale communications systems. Indeed, it has been suggested that optical systems utilizing VCSELs may eventually replace many systems that currently rely upon copper wires to transmit electrical data signals. The advantages of optical communications systems over electrical systems commonly include high bandwidth and low signal loss which often results as optical data signals travel through the length of the fiber. Moreover, several optical fibers may be bundled together in a "fiber array" to form a communications channel that is capable of transmitting multiple signals simultaneously.
An important element of any optical communications system is a method of coupling light emanating from a light source into the waveguide. Typically, a laser light source is coupled into an optical fiber in a "header block" arrangement. The most commonly used form of header uses the well-known "butt coupling" method shown in FIG. 1. "Butt coupling" involves positioning the laser so that light is directly emitted into an end of the optical fiber. Typically, a substrate made of silicon, ceramic or another material supports the laser and at least a portion of the optical fiber. The "butt coupling" method is particularly suited for use with edge emitter lasers that emit photons in an elliptical pattern, with the vertical axis of the pattern being longer than the horizontal axis.
A common practice is to cut a groove into the substrate to support the optical fiber. Although the groove often prevents lateral movement of the fiber, it also typically increases the difficulty in aligning the fiber with the light source since the elliptical pattern of light emanating from the edge emitter is substantially narrow in the lateral direction. The grooves must therefore be precisely placed or else significant amounts of light can be lost, thus degrading the transmitted optical signals.
Often, the intensity of the light emitted by the laser is not constant over time.
For example, environmental effects such as temperature or humidity changes can affect the performance of the laser. To compensate for variations in laser output, it is frequently desirable to monitor the intensity of the light emitted by the laser. The intensity of the light is proportional to the output power of the laser, and the stability of the laser can be greatly improved by using the monitoring signal as feedback into the light source controls. This feedback signal is obtained by measuring the output intensity of the laser by a detector such as a photodiode and providing this signal to a well-known electronic feedback circuit that provides a drive signal to the laser as shown in FIG. 1.
Typically, it is impractical to measure the direct output of the laser, since an intensity detector cannot be placed between the laser and the optical fiber without significantly degrading the amount of light impinging upon the fiber. Many lasers, including edge emitting lasers, emit light at both the front and back ends of the lasing cavity, commonly called the front and back facets. The front facet is generally the primary output of the laser, with substantially fewer photons emanating from the back facet. Still, the light emanating from the back facet can provide an input to an intensity monitor in a feedback system. Using the back facet as an input to an intensity monitor, however, often results in two distinct disadvantages. First, the power output from the back facet is not always directly proportional to the light which enters the fiber from the front facet, since the relative intensities of light emanating from the front and back facets can vary over time. Moreover, VCSELs do not typically have a back facet. Therefore, it is not desirable to use a VCSEL in a butt-coupling arrangement with a power monitor.
U.S. Pat. No. 5,163,113, issued Nov. 10, 1992 to Paul Melman, which is incorporated herein by reference, generally discloses a second form of a header block arrangement that includes an edge emitting laser configured to provide light in a vertical direction. As can be seen in FIG. 2A, an untreated optical fiber is cleaved at about a 45 degree angle, and this cleave is positioned directly above an edge-emitting laser attached to a submount block so that emitted light substantially impinges upon the inner face of the cleaved end of the fiber. Alternatively, an edge-emitting laser directs light horizontally toward a mirror, and the mirror reflects light vertically toward the fiber as shown in FIG. 2B. Because the optical fiber is untreated, light from the laser is substantially reflected by the cleaved end into the longitudinal axis of the fiber. This arrangement provides several advantages over the butt-coupling method. Most notably, the header is suitable for use with vertically-emitting VCSEL lasers. Moreover, the cleaved fiber approach allows improved fiber/light source alignment over the butt-coupling approach. However, this approach often exhibits a marked disadvantage in that monitoring the output intensity of the laser light source is impractical. Moreover, the elements required to implement this method with an edge emitter (namely the submount block in FIG. 2A or the mirror structure in FIG. 2B) are cumbersome to manufacture.
Accordingly, there exists a need for an optical header arrangement that efficiently couples light from an emitter source into an optical fiber while providing a substantially accurate measure of the intensity of the emitted light. Moreover, there exists a need for such a header to incorporate VCSEL lasers, to handle bi-directional optical communications, and to support arrays of fibers that are used in communications systems. This header should contain minimal components to simplify manufacturing.